hennis et al genetic factors associated with exercise performance in atmospheric hypoxia 2015 online...

7/23/2019 Hennis Et Al Genetic Factors Associated With Exercise Performance in Atmospheric Hypoxia 2015 Online Version

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S Y S T E M A T I C R E V I E W

Genetic Factors Associated with Exercise Performancein Atmospheric Hypoxia

Philip J. Hennis Alasdair F. ODoherty

Denny Z. H. Levett Michael P. W. Grocott

Hugh M. Montgomery

The Author(s) 2015. This article is published with open access at Springerlink.com

Abstract

Background and Objective Natural selection has beenshown to have enriched the genomes of high-altitude native

populations with genetic variants of advantage in this

hostile hypoxic environment. In lowlanders who ascend to

altitude, genetic factors may also contribute to the sub-

stantial interindividual variation in exercise performance

noted at altitude. We performed a systematic literature

review to identify genetic variants of possible influence on

human hypoxic exercise performance, commenting on the

strength of any identified associations.Criteria for considering studies for this review All stud-

ies of the association of genetic factors with human hy-

poxic exercise performance, whether at sea level using

nitrogen dilution of oxygen (normobaric hypoxia), or at

altitude or in low-pressure chambers (field or chamber

hypobaric hypoxia, respectively) were sought for review.

Search strategy for identification of studies Two elec-

tronic databases were searched (Ovid MEDLINE, Embase)

up to 31 January 2014. We also searched the reference lists

of relevant articles for eligible studies. All studies pub-

lished in English were included, as were studies in any

language for which the abstract was available in English.

Data collection and analysis Studies were selected and

data extracted independently by two reviewers. Differences

regarding study inclusion were resolved through discus-

sion. The quality of each study was assessed using a

scoring system based on published guidelines for con-

ducting and reporting genetic association studies.

Results A total of 11 studies met all inclusion criteria and

were included in the review. Subject numbers ranged from

20 to 1,931 and consisted of healthy individuals in all

cases. The maximum altitude of exposure ranged from

2,690 to 8,848 m. The exercise performance phenotypes

assessed were mountaineering performance (n = 5), run-

ning performance (n = 2), and maximum oxygen con-

sumption (_VO2max) (n = 4). In total, 13 genetic

polymorphisms were studied, four of which were associ-

ated with hypoxic exercise performance. The adenosine

monophosphate deaminase (AMPD1) C34T (rs17602729),

beta2-adrenergic receptor (ADRB2) Gly16Arg single nu-

cleotide polymorphism (SNP) (rs1042713), and androgen

receptor CAG repeat polymorphisms were associated with

Electronic supplementary material The online version of this

article (doi:10.1007/s40279-015-0309-8 ) contains supplementarymaterial, which is available to authorized users.

P. J. Hennis (&) D. Z. H. Levett M. P. W. Grocott

H. M. Montgomery

UCLH NIHR Biomedical Research Centre, Institute of Sport and

Exercise Health, University College London Centre for Altitude

Space and Extreme Environment Medicine, 170 Tottenham

Court Road, London W1T 7HA, UK

e-mail: [email protected]

A. F. ODoherty

Department of Sport, Health and Exercise Science,

University of Hull, Hull, UK

D. Z. H. Levett

M. P. W. GrocottIntegrative Physiology and Critical Illness Group, Clinical

and Experimental Sciences, Sir Henry Wellcome Laboratories,

Faculty of Medicine, University of Southampton, University

Hospital Southampton NHS Foundation Trust, Southampton, UK

D. Z. H. Levett M. P. W. Grocott

Anaesthesia and Critical Care Research Unit, University

Hospital Southampton NHS Foundation Trust, Southampton, UK

D. Z. H. Levett M. P. W. Grocott

NIHR Southampton Respiratory Biomedical Research Unit,

Southampton, UK

1 3

Sports Med

DOI 10.1007/s40279-015-0309-8
http://dx.doi.org/10.1007/s40279-015-0309-8http://dx.doi.org/10.1007/s40279-015-0309-8


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altitude performance in one study, and the angiotensin

I-converting enzyme (ACE) insertion/deletion (I/D)

(rs4646994) polymorphism was associated with perfor-

mance in three studies. The median score achieved in the

study quality analysis was 6 out of 10 for casecontrol

studies, 8 out of 10 for cohort studies with a discrete out-

come, 6 out of 9 for cohort studies with a continuous

outcome, and 4.5 out of 8 for genetic admixture studies.Conclusion The small number of articles identified in the

current review and the limited number of polymorphisms

studied in total highlights that the influence of genetic

factors on exercise performance in hypoxia has not been

studied in depth, which precludes firm conclusions being

drawn. Support for the association between the ACE-I al-

lele and improved high-altitude performance was the

strongest, with three studies identifying a relationship.

Analysis of study quality highlights the need for future

studies in this field to improve the conduct and reporting of

genetic association studies.

Key Points

The size and scope of the literature regarding the role

of genetics on high-altitude exercise performance is

limited.

The association between the angiotensin

I-converting enzyme (ACE) insertion (I) allele and

improved high-altitude performance has the

strongest support.

The volume and quality of the literature limits firm

conclusions being made and needs to be addressed in

future studies.

1 Introduction

With ascent to altitude, barometric pressure falls, and with

it the partial pressure of inspired oxygen (PiO2). Despite

diverse adaptive responses, the arterial partial pressure of

oxygen (PaO2) also falls. Such hypobaric hypoxia impairs

physical performance and can cause pathology (acute

mountain sickness [AMS]), which may be fatal (e.g. high-

altitude pulmonary oedema [HAPE] or high-altitude cere-

bral oedema [HACE]).

Longstanding communities of high-altitude residents

exist worldwide, most notably on the Tibetan Plateau and

Andean Altiplano, but also in the area of the Great Rift

Valley in Ethiopia. Natural selection has enriched the

genomes of high-altitude native populations, conferring

advantage in this hostile hypoxic environment [1]. Mean-

while, increasing numbers of native lowlanders travel to

high altitudes for recreation or work. Amongst these low-

land populations, similar genetic factors may contribute to

the substantial inter-individual variation in exercise per-

formance and wellbeing observed at altitude [2,3]. Whilst

attempts have been made to identify genetic variants as-sociated with improved high-altitude performance, the re-

sults of such studies have yet to be collated. We thus set out

to systematically review the published literature and

identify genetic variants of influence on human hypoxic

exercise performance, commenting on the strength of any

identified associations.

2 Methods

2.1 Search Strategy

2.1.1 Criteria for Considering Studies for this Review

All studies of the association of genetic factors with human

hypoxic exercise performance, whether at sea level using

nitrogen dilution of oxygen (normobaric hypoxia), or at

altitude or in low-pressure chambers (field or chamber

hypobaric hypoxia, respectively) were sought for review.

Altitude was defined as a real or simulated (normobaric

hypoxia) elevation ofC2,000 m above sea level. Perfor-

mance measures were defined as any measure of aerobic

exercise capacity (such as _VO2max or _VO2peak) or of

exercise performance (e.g. a timed physical challenge).Studies were included if they evaluated exercise perfor-

mance under conditions of hypoxia or changes in sea-level

performance following hypoxic exposure. Casecontrol

studies were included where cases were successful per-

formers at[2,000 m, and where appropriate control groups

(by race and by similar altitude of origin [500 m]) were

utilised. Only human studies and full primary research

papers (e.g. not a conference abstract, letter to the editor, or

review) were eligible for inclusion. Studies of high-altitude

population genetics per se were excluded unless

specifically assessing a physical performance phenotype.

Research articles were excluded if they did not seek toidentify genetic factors and their relationship with hypoxic

exercise performance.

2.1.2 Search Strategy for Identification of Studies

The following electronic databases were searched: Ovid

MEDLINE Daily Update \31 January 2014[, Ovid

OLDMEDLINE\1946 to 1965[, Ovid MEDLINE In-

Process and Other Non-Indexed Citations, Ovid

P. J. Hennis et al.

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MEDLINE \1946 to present[ and Embase Clas-

sic ? Embase \1947 to 31 January 2014[. Ovid MED-

LINE is produced by the National Library of Medicine and

indexes information from approximately 5,600 journals in

the fields of biomedicine and related fields. Embase is an

Elsevier database that indexes biomedical and pharmaco-

logical bibliographic records from articles published in

more than 7,500 peer-reviewed journals. The search in-cluded all studies published in English and studies in any

language for which the abstract was available in English.

No other limits were applied.

We developed a search strategy based on the patient or

population, intervention, comparison, outcome (PICO)

framework. However, the types of study we sought do not

conform exactly to the PICO format, which could not

therefore be fully adhered to. In particular, all populations

were eligible for inclusion, making the patient or

population parameter redundant. In order to formulate a

research question, each search contained one term from

each of the following three parameters (intervention,comparison, and outcome) with each term linked by the

Boolean operator AND. The full search strategy run in

the various databases can be found in Electronic Supple-

mentary Material Appendix S1 and Appendix S2. In brief,

the key terms used in the search are listed hereafter, and

these were also used to identify synonyms and related

terms (which were also searched for):

Intervention: Altitude, anoxia, or hypoxia.

COMPARISON: Gene frequency, genotype, polymor-

phism, haplotype, single nucleotide polymorphism, or

genetic linkage. Outcome: Exercise tolerance, exercise test, exercise,

athletic performance, performance, mountaineering,

summit, physical endurance, endurance.

2.1.3 Searching Other Resources

In addition to electronic databases, we searched bibli-

ographies of included articles for research papers. Research

papers with titles that indicated that the study was in the

scope of the current review were retrieved and underwent

the formal review process.

2.1.4 Data Collection and Analysis

In the first stage of the review process, two authors (PJH,

AFO) independently inspected all citations from the

searches (using information contained in the title, key-

words and abstract) to identify candidate articles for which

full texts were retrieved. If any ambiguity arose regarding

the inclusion or exclusion of an article due to the absence

of key information, the full-text article was obtained. In the

second stage, the authors (PJH, AFO) independently ap-

plied the inclusion and exclusion criteria to the candidate

full-text articles. At both stages of the process, differences

regarding study inclusion were resolved through

discussion.

2.1.5 Data Extraction and Management

From each source document, the lead author (PJH) ex-

tracted data including study identification information,

number of participants in each group, demographic in-

formation (including the sex, age and ethnicity, race or

origin of the participants), magnitude of hypoxic expo-

sure (altitude or hypoxic exposure), polymorphism and/or

gene or region being studied, gene and allele frequen-

cies, probability values from comparisons, measure of

performance, and the information required to assess the

quality of each study (see Table1). Studies were

separated according to their primary study design (case

control or cohort). Odds ratios (ORs) were determinedfor studies with categorical outcomes variables using an

additive model.

2.2 Study Quality Analysis

The quality of each study was assessed using a scoring

system (Table1) adapted from Clark and Baudouin [4],

which was based on published guidelines for conducting

and reporting genetic association studies. Different aspects

of the study were scored 1 if the information was present or

0 if it was not. These were then summed to give a final

score that ranged from 0 to 10 for casecontrol studies, 09

for cohort studies, and 08 for genetic admixture studies.

We modified the approach of Clark and Baudouin [4] in

relation to sample size and power calculations. Studies

were deemed adequate in this respect if a power calculation

was reported, but we did not perform a power calculation

for studies where this was not reported. The criteria used

for considering the quality of the control group were also

adapted. In the original system by Clark and Baudouin [4],

a study that used a control group from a previously pub-

lished study would need to describe the population in

sufficient detail to meet the study quality criteria to score.

Due to word restrictions imposed by publishers and the

need to avoid repetition, we extended this. In studies that

provided a reference to a control population, the referenced

study was retrieved and the control group analysed ac-

cording to the original criteria. We also altered the methods

used to assess the reproducibility of genotyping to allocate

a score both to studies that provided their own validation

results and to those that provided a reference to a study

using the same procedures that included validation

information.

Genetic Factors and Exercise Performance in Hypoxia

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3 Results

3.1 Results of the Search

The initial search identified 307 articles (see Fig. 1). After

removing duplicate articles, 213 remained for the first

evaluation. Following review of the title, abstract and

keywords of these records, 12 candidate articles were

identified that met the predefined criteria, and full-text

versions were obtained for further review. Four candidate

articles did not meet the review inclusion criteria and were

therefore rejected, leaving eight eligible articles for ap-

praisal [512]. Examination of the full-text version of these

articles and their bibliographies identified three additional

eligible articles [1315]. A total of 11 articles were in-

cluded in the final analysis.

3.2 Description of Included Studies

The 11 studies reviewed can be separated into those

comparing the frequency of genotypes or alleles between

two groups (casecontrol) (Table2)[6,15]; cohort studies

where participants were separated according to task suc-

cess or failure and where genotype or allele frequencies

were compared between groups (Table3) [810]; and

cohort studies comparing the difference in performance

measures (continuous variable) between individuals

grouped by genotype or allele (Table4) [6,7,11,12]. Two

studies used a different approach, and examined whether

Spanish compared with Quechua genetic admixture ex-

plained differences in _VO2max at high altitude [13, 14].

According to the location of the corresponding author,

studies were from the UK (n = 4), the USA (n = 3),

Greece (n = 1), New Zealand (n = 1), South Africa

(n = 1), and China (n = 1). The number of subjects in

each study varied from 20 to 1,931, and consisted of

healthy males in four studies [1215] and healthy males

and females in six studies [5, 711], with sex of the par-

ticipants not stated in one study [6]. The maximum altitude

of exposure ranged from 2,690 to 8,848 m. The exercise

performance phenotype assessed was mountaineering per-

formance in five studies [6,810,15], running performance

in two studies [7, 11] and maximum oxygen consumption

in four studies [5, 1214]. In total, 13 genetic polymor-

phisms located within ten different genes were studied,

excluding those used to estimate genetic admixture by

Brutsaert and colleagues [13, 14]. The most studied gene

was that encoding the angiotensin I-converting enzyme

(ACE), which was studied in seven separate papers. The

names of all the genes studied, excluding those used to

Table 1 Study quality scoring assessment system

Item Criterion

1. Control group Was the control/comparison group equal or larger in size than the case group, and was it

described in such a way that it could be replicated;

stated or inferred that the ethnicity of the control group was not different to that of the case group.

If the control group from a previous study was used and referenced, the referenced study was retrieved and the

control group analysed as above. No score was assigned to cohort studies for this item2. HardyWeinberg

equilibrium

Were the groups included in the study assessed for to determine whether they were in HardyWeinberg

equilibrium?

3. Case group/whole

group

Is the definition of the case group adequate to allow replication?

For cohort studies, is the description of the whole group sufficient for replication?

4. Primer Were the primer sequences provided or was a reference to these given?

5. Reproducibility of

genotyping

Was the description of genotyping methods sufficient to allow replication or was a reference providing this

information given? And, was the validity of the genotyping technique checked by performing a second assay

technique, by validating the accuracy of the assay used, or was a reference to a validation study given?

6. Blinding Were genotyping staff blinded from group allocation or phenotypic data?

7. Power calculation Was a power calculation performed either prospectively or retrospectively?

8. Statistics Were major findings presented with well described tests of significance includingpvalues, odds ratios or confidence

intervals?

9. Corrected statistics When more than one genetic polymorphism was studied, were statistical corrections made to account for the

increased risk of a false positive?a One-tailed significance testing was also scored as zero. Those testing one

genetic polymorphism were scored as one

10. Independent

replication

Was a secondary confirmatory study performed, or does the study specifically state it is being performed to confirm

results from an earlier study?

Items were scored 1 for yes and 0 for noa If the study only investigated one genetic polymorphism, the question was scored as 1

P. J. Hennis et al.

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indicate Quechua and Spanish ancestry by Brusaert and co-

workers [13, 14], are listed in Table5 along with the

symbols and the chromosomal location. Results are pre-

sented according to the genetic polymorphism of interest,

then by phenotype of interest (given sub-heading where

necessary), and are ordered based on the maximum altitude

studied.

3.3 Angiotensin-Converting Enzyme (ACE) Insertion/

Deletion (I/D) Polymorphism (rs4646994)

Seven studies investigated the association of the ACE I/D

(rs4646994) polymorphism with exercise performance at

altitude. Two were casecontrolled studies that compared

genotype or allele frequencies between high-altitude

mountaineers and a control population (Table2). Five

cohort studies were also conducted; in three of these, study

participants were separated according to task success or

failure, and genotype and/or allele frequencies were com-pared between groups (Table3), whilst two studies com-

pared performance phenotype (as a continuous variable

[e.g. time and _VO2max]) between those of different

genotype (Table4).

3.3.1 Mountaineering Performance

In a prospective study, Tsianos et al. [10] reported the ACE

I-allele to be associated with successful ascent of Mont

Blanc (4,807 m). ACE genotype distribution varied be-

tween Caucasian climbers who successfully summited Mt

Blanc (n = 183) and those who tried and failed (n = 12)

(p = 0.048) (OR 3.41), the I allele being more prevalent in

those who summited than in those who did not (0.47 vs

0.21,p = 0.01) (Table3). The proportion of those of each

genotype who successfully summited was 87.7, 94.9 and100 % for the DD, ID, and II genotypes, respectively.

Kalson et al. [8] prospectively studied Caucasian trek-

kers attempting to climb Mt Kilimanjaro (5,895 m), par-

ticipants being separated by ascent rate (4 days [direct]

vs. 5 days [slow]). Genotype distribution did not differ by

success rate in the slow group (II: 9 [0.22], ID: 18 [0.44],

DD: 14 [0.34] successful versus II: 9 [0.22], ID: 22 [0.54],

DD: 10 [0.24] unsuccessful, p = 0.54; I allele frequency

0.44 vs. 0.49, respectively; OR 0.82). In fast ascent

subjects, ACE genotype distribution was II: 6 (0.30) vs. ID:

11 (0.55) vs. DD: 3 (0.15) in those who were successful

and II: 0 (0.00) vs. ID: 10 (0.71) vs. DD: 4 (0.29) in thosewho failed (p = 0.09). I-allele frequency for those who

were successful versus those who failed was 0.58 vs. 0.36

(OR 2.44) (Table 3).

Montgomery et al. [15] studied ACE genotype in 25

elite British high-altitude mountaineers who had frequently

ascended beyond 7,000 m without supplementary oxygen.

When compared with 1,906 British controls (free from

clinical cardiovascular disease), the I allele was found to be

over-represented in cases (p = 0.003), as was the ACE II

Arcles idenfied through database searching

(n = 307)

Arcles for inial inspecon aer duplicates

removed (n = 213)

Arcles included in the review from the

database search (n = 8)

Full text arcles reviewed (n = 12)

Total number of arcles included in review

(n = 11)

Arcles added aer

bibliography search (n = 3)

Arcles excluded following

inial review of tle, abstract

and keywords (n = 201)

Arcles excluded following

full text review (n = 4)

Fig. 1 Search methodology and results


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Table2

Exerc

iseperformance

inatmospher

ichypoxiaan

dcand

idategenes;case-c

ontrolstud

ies

Gene

Polymorph

ism

Case

Control

pValue

OR

References

N (no.of

males

)Populat

ion

Frequency

n(no.ofmales

)

Popu

lation

Frequency

ACE

I/D (r

s4646994)

25(25)

Elite

Britishmountaineers

II:

12(0

.48)

ID:

11(0

.44)

DD:

2(0

.08)

1,906(1

,906)

Britis

hfree

from

CV

dise

ase

II:

(0.2

6)

ID:

(0.5

0)

DD:

(0.2

4)a

0.02

2.43

Montgomery

etal

.[15]

5(NS)

Bulgarianel

ite

high-a

ltitude

mountaineers

II:

1(0

.20)

ID:

4(0

.80)

DD:

0(0

.00)

72(NS)

Athle

tic

Bulgarian

stud

ents

II:

8(0

.11)

ID:

41

(0.5

7)

DD:

23

(0.3

2)

0.024

2.29

Djarovaetal

.[6]

ACTN3

R577X

(rs1

815739)

5(NS)

Bulgarianel

ite

high-a

ltitude

mountaineers

RR:

1(0

.20)

RX:

3(0

.60)

XX:1

(0.2

0)

72(NS)

Athle

tic

Bulgarian

stud

ents

RR:

30

(0.4

2)

RX:

32

(0.4

4)

XX:

10

(0.1

4)

0.032b,

c

1.77

Djarovaetal

.[6]

AMPD1

G/A (r

s17602729)

5(NS)

Bulgarianel

ite

high-a

ltitude

mountaineers

CC:

3(0

.60)

CT:

1(0

.20)

TT:

1(0

.20)

72(NS)

Athle

tic

Bulgarian

stud

ents

CC:

53

(0.7

4)

CT:

19

(0.2

6)

TT:

0(0

.00)

0.003b,

c

2.82

Djarovaetal

.[6]

CVcard

iovascular

,NSnotstated,O

Rod

dsratio

a

Valuesta

ken

fromsu

bsequentpa

perus

ingthesamepopu

lation

b

Statist

icforal

lelecomparisons

c

Statist

icalanalys

isappearsto

be

incorrect

P. J. Hennis et al.

1 3


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Table3

Exerc

iseperformance

inatmospher

ichypoxiaan

dcand

idategenes:co

hortstud

iesseparate

dbysumm

itsuccess

Gene

Polymorph

ism

Tas

ksucce

ss

Tas

kfailure

pV

alue

OR

References

n(no.ofm

ales

)

Populat

ion

Frequency

n(no.

of males

)

Populat

ion

Frequency

ACE

I/D (r

s4646994)

183(NS

whole

popu

latio

n:83%

male)

Caucasianssuccessful

in

summ

iting

Mt

Blanc

(4,8

07m

)

II:

40(0

.22)

ID:

93

(0.5

1)

DD:

50

(0.2

7)

12 (N

S)

Caucasianswh

ofailedtosumm

it

Mt

Blanc

(4,8

07m

)

II:

0(0

.00)

ID:

5(0

.42)

DD:

7(0

.58)

0.048

3.41

Tsianos

etal

.[10]

92(NS

w

hole

popu

latio

n:90%

male)

Caucasiansan

dAsian

mountaineerssuccessful

in

summ

iting[

8,000m

II:

30(0

.33)

ID:

41

(0.4

5)

DD:

21

(0.2

3)

57 (N

S)

Caucasiansan

dAsianmountaineers

unsuccessful

in

summ

iting[

8,000m

II:

3(0

.06)

ID:

28(0

.60)

DD:

16

(0.3

4)

0.003

2.15

Thompson

etal

.[9]

41(NS

w

hole

popu

latio

n:60%

male)

Success

fullyascendedto

5,895

m

withaslowascentprofi

le

II:

9(0

.22)

ID:

18

(0.4

4)

DD:

14

(0.3

4)

41 (N

S)

Unsuccess

fulinanattemptto

summ

it5,895mw

ithaslow

ascentprofile

II:

9(0

.22)

ID:

22(0

.54)

DD:

10

(0.2

4)

0.54

0.82

Kalson

etal

.[8]

20(NS

w

hole

popu

latio

n:60%

male)

Success

fullyascendedto

5,895

m

witha

fastascentprofi

le

II:

6(0

.30)

ID:

11

(0.5

5)

DD:

3(0

.15)

14 (N

S)

Unsuccess

fulinanattemptto

summ

it5,895mw

itha

fastascent

profi

le

II:

0(0

.00)

ID:

10(0

.71)

DD:

4(0

.29)

0.09

2.44

Kalson

etal

.[8]

ACEangiotensin

1-convertingenzy

me,I/Dinsert

ion/

deletion

,NSnotstated,O

Rod

dsratio


1 3


8/17


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genotype (proportion 0.48 vs. 0.23 in cases vs. controls,

respectively) (OR 2.43) (Table 2). The DD genotype was

not present in any individual who had previously ascended

beyond 8,000 m.

Djarova et al. [6] also reported I allele prevalence to begreater in five elite Bulgarian mountaineers (all climbed

above 7,500 m without supplementary oxygen) than in 72

Bulgarian student controls (0.60 vs. 0.41, respectively;

p = 0.002) (OR 2.29). ACE genotype distribution differed,

largely due to an excess of ID genotype in the elite group

(p = 0.024) (Table2). The small case population (n = 5)

limits confidence in this finding. Further, we were unable to

replicate their statistical analysis: using the ACE allele

frequencies and population numbers stated in the paper,

and applying Fishers exact test (as the authors did) in a

commonly used statistical package (SPSS 21 [IBM, Ar-

monk, NY, USA]), we were unable to demonstrate a sta-tistically significant difference in ACE allele frequencies

between the two groups (p = 0.203). The same held true

for the two other polymorphisms they studied, which were

the alpha actinin-3 ACTN3 R577X (rs1815739) and the

adenosine monophosphate deaminase (AMPD1) C34T

polymorphisms (rs17602729).

Thompson et al. [9] extended the observations of Tsia-

nos et al. [10], Kalson et al. [8] and Montgomery et al. [15]

to individuals attempting to climb beyond 8,000 m, also

noting the maximum altitude attained by each individual

and the reason for failure. Once again, in a population of

Caucasian and Asian mountaineers, ACE genotype distri-bution differed between successful summiteers (n = 92)

and those who failed (n = 57) (OR 2.15) (Table3)

(p = 0.003). Both II genotype and the I allele were more

prevalent in the successful group (II genotype 0.33 vs. 0.06

[p = 0.003]; I allele, 0.55 vs. 0.36 for success vs failure,

respectively). Maximum altitude attained (as a continuous

variable) was also related to ACE I allele, being 8,079 vs.

8,107 and 8,559 m for those of DD, ID and II genotype,

respectively (allele comparison, p = 0.0023).

3.3.2 Maximum Oxygen Uptake ( _VO2max) at Altitude

Bigham et al. [5] reported the relationship of ACE geno-

type with _VO2max at 4,338 m in Peruvian high-altitude

native residents (n = 74) and in Peruvian low-altituderesidents whose family originate from high altitude

(n = 68). _VO2max did not differ by genotype, even when

other cofactors, such as altitude of residence, were ac-

counted for (Table4).

3.3.3 Sea-Level Running Performance

Hinckson et al. [7] studied whether improvement in sea-

level exercise performance following hypoxic exposure

(2,5003,500 m) was related to ACE genotype (Table 4).

This study was primarily designed to test the efficacy of

hypoxic training for improving running performance. Itincluded a study group whose ten participants were ex-

posed to 4 weeks of hypoxia for [10 h per day, and ten

controls who had no hypoxic exposure. In eight par-

ticipants (ethnicity not disclosed) in the study group, the

relationship between improvements in running perfor-

mance following hypoxic training with ACE I/D genotype

was sought (Table 3). However, neither numerical results

by genotype nor statistical analysis are reportedalthough

a figure suggests a lack of genotype association.

3.4 ACTN3 R577X (rs1815739)

Two studies tested whether an association was present

between the ACTN3 R577X polymorphism and high-alti-

tude exercise performance. Tsianos et al. [11] included the

ACTN3 RX polymorphism as one of 11 polymorphisms (in

eight genes) for which an association was sought with

performance in the Mt Olympus Marathon (maximum

altitude of 2,690 m). In all, no such association was iden-

tified in 438 participants over 2 years of racing. The dis-

tribution of the ACTN genotypes in the small elite high-

Table 5 Symbols, names, and chromosomal location of all genes identified

Gene Name Location

ACE Angiotensin 1-converting enzyme 17q23

ACTN3 Actinin, alpha 3 11q13q14

ADRB2 Adrenergic, beta 2, receptor 5q31q32

AMPD1 Adenosine monophosphate deaminase 1 1p13

APOE Apolipoprotein E 19q13.2

AR Androgen receptor Xq1112

BDKRB2 Bradykinin receptor B2 14q32.1q32.2

PPARA Peroxisome proliferative activated receptor, alpha 22q13.31

PPARD Peroxisome proliferative activated receptor, delta 6p21.2p21.1

PPARGC1A Peroxisome proliferative activated receptor, gamma, coactivator 1, alpha 4p15.1


1 3


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altitude mountaineer cohort (n = 5) tested by Djarova

et al. [6] was RR 1 (0.20), RX 3 (0.60), and XX 1 (0.20)

compared with RR 30 (0.42), RX 32 (0.44) and XX 10

(0.14) in the control group (n = 72). The allele frequencies

between the two groups were different (p = 0.032), with a

relative excess of the X allele in the mountaineers (OR

1.77) (Table2). However, as discussed in the previous

section, the statistical approach of Djarova et al. [6] was ofquestionable validity. Using the allele frequencies and

subject numbers from the paper [6], we found no difference

in the ACTN RX allele distributions between the two

populations (p = 0.501).

3.5 AMPD1 C34T (rs17602729)

In the study by Tsianos et al. [11] (see previous section),

the AMPD C34T polymorphism was not associated with

Mt Olympus Marathon performance for the whole group

studied, but subgroup analysis revealed a modest asso-

ciation between the T allele (referred to as the A allele inTsianos et al. [11]) and better performance in athletes

whose preferred sport was running (event completion time,

p = 0.021; best ever time, p = 0.03). Djarova et al. [6]

found that the frequency of the T allele was higher in

mountaineers than in controls (p = 0.032; OR 2.82)

(Table2). However, as stated in the previous two results

sections, the statistical analysis in this study is incorrect;

using the reported allele frequencies in a Fishers test, we

found no difference in the distribution of the allele of the

AMPD1 C34T polymorphism between the elite moun-

taineers and controls (p = 0.155).

3.6 Other Polymorphisms

Ten other polymorphisms were studied. Of nine studied by

Tsianos et al. [11] (listed in Table 4), the beta2-adrenergic

receptor (ADRB2) Gly16Arg single nucleotide polymor-

phism (SNP) (rs1042713) was associated with performance

in the Mt Olympus Marathon. In the study by Tsianos et al.

[11], the Arg allele was associated with a shorter time to

complete the race amongst people who chose running as

their preferred exercise mode (p = 0.015), and with best

ever Mt Olympus Marathon time for those who had com-

pleted the event multiple times (p = 0.003). Meanwhile,

Wang et al. [12] studied the association between the an-

drogen receptor (AR) CAG repeat polymorphism and

_VO2max changes in 65 unrelated Han Chinese college

athletes following a 30-day bout of intermittent hypoxic

exposure, consisting of 10 h a day of sleeping at 14.8

14.3 % O2 (simulating 2,8003,000 m altitude), 30-minute

exercise bouts at 15.4 % O2 (simulating 2,500 m altitude)

three times per week, and additional sea-level training.

They found that individuals with the lowest number of

CAG repeat units (B21) had a greater increase in _VO2max

following hypoxic training than those with a higher number

of repeats ([21) (D_VO2max; B21 CAG repeats 11.9 %

increase,[21 CAG repeats 3.6 % increase; p = 0.004).

3.7 High-Altitude Population Ancestry

Two papers from the same group measured a number of

physiological variables, including _VO2max, at altitude in

Peruvian males of mixed Quechua and Spanish origins [13,

14]. Both sought to determine whether individuals with

stronger Quechua (historical long-term high-altitude resi-

dent) origins had favourable physiological responses to

altitude. We considered excluding these two papers as,

strictly speaking, they do not meet one of the inclusion

criteria: to seek to identify genetic factors. However, we

did include them given that they studied exercise perfor-

mance at altitude and separated individuals based on theirgenetic profile. The first, published in 2004, comprised two

groups from Peru; a group resident in Lima (sea level) and

a group resident in Cerro de Pasco (4,388 m) [13]. No

association was found between _VO2max values at an alti-

tude of 4,388 m, and the degree of Quechua, Spanish and

African ancestry, as indicated by 22 genetic markers. The

second study [14] used the same lowland population and

the same genetic profiling as the 2004 study [13] (con-

firmed with primary author of the manuscripts via email

correspondence) and analysed the influence of genetic ad-

mixture on the magnitude of the _VO2max decrement from

sea level to high altitude (data not presented in the sameauthors 2004 paper). After controlling for other covariates,

genetic admixture explained a significant proportion of the

variation (b -0.205) in the decrement in _VO2max from sea

level to high altitude (p = 0.041), with those with a higher

Quechua (and lower Spanish) genetic ancestry maintaining

sea-level _VO2max values to a greater degree.

3.8 Study Quality

As the method of assessing study quality is not validated

and may only provide a guide to the quality of reporting of

the research, we have not commented on each study indi-

vidually. Instead, we aim to identify trends in the available

literature where the conduct and/or reporting of research

could be improved. The median score achieved in the study

quality analysis was 6 out of 10 for casecontrol studies, 8

out of 10 for cohort studies with a discrete outcome, 6 out

of 9 for cohort studies with a continuous outcome, and 4.5

out of 8 for genetic admixture studies (Table6). The study

quality score ranged considerably between studies, for

P. J. Hennis et al.

1 3


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example in cohort studies with a continuous outcome the

total score ranged from 2 to 7 out of 9.

In the casecontrol studies, the descriptions of the case

groups were adequate, with both the studies given a score

of 1 [6,15]. However, only one of the control groups was

deemed adequate [15], with the other study failing to

provide adequate information about whether the level of

relatedness of participants was controlled [6]. The majorityof studies in which testing for HardyWeinberg (HW)

equilibrium was appropriate did so (seven of eight). The

one study that did not test for HW equilibrium had very

small subject numbers (n = 8) and so it would have pro-

vided very little information [7]. In the other three studies,

two investigated whether genetic admixture was associated

with performance. This approach utilised polymorphisms

that differed according to ancestry, meaning that testing for

HW equilibrium would be uninformative [13, 14]. In the

other study, the polymorphism was located on the X

chromosome, and the populations were male, so genotype

distribution would not conform with HW equilibrium,evidence of which was thus not sought [12].

Three study quality criteria relate to genotyping methods

and reporting: providing the sequence of primers, demon-

strating the reproducibility of the genotyping methods, and

adequately blinding the genotyping staff from the pheno-

type details. Most studies (7 of 11) provided the details of

the primer sequence (or a reference to them), one stated

that this would be made available on request [11], two gave

reference to a website that could be used to identify them

[13, 14], and one failed to provide any information [7].

Eight studies stated that they had validated their genotyp-

ing methods or cited a reference for a validation study,

while only two studies stated that their genotyping staff

were blinded from the phenotype data.

The statistical merit of each paper was analysed in three

of the study quality criteria. These analysed whether ap-

propriate statistical tests were performed, whether adjust-

ments were made to account for multiple comparisons in

studies analysing more than one polymorphism, and whe-

ther power calculations were made. Ten of the 11 studies

used appropriate formal methods of comparison to estab-

lish whether difference were present between groups. Only

one study did not use a formal test of significance, and this

was probably justified because the small number of par-

ticipants would have precluded any meaningful results

being obtained [7]. Only one study analysed several poly-

morphisms and failed to make adjustments for multiple

comparisons [6]. Four of the 11 studies conducted sample

size calculations [6, 8,11,13], with one study performing

the calculation prior to conducting the study [11].

No article included in the review provided both a study

identifying a potential polymorphism and a study validat-

ing the original finding. Following the original paper

analysing the association between the ACE I/D polymor-

phism and mountaineering performance by Montgomery

et al. [15], several other papers analysed the association

between this polymorphism and highly related phenotypes

[6, 810]. Furthermore, one study replicated polymor-

phisms related to endurance performance at sea level using

an endurance race at altitude as the phenotype [11].

4 Discussion

4.1 Main Findings

This review identified four genetic polymorphisms asso-

ciated with hypoxic exercise performance: AMPD1 C34T,

ADRB2 GA, AR CAG repeat, and the ACE I/D poly-

morphisms. Support for the association between the ACE I

allele and high-altitude performance was the strongest,

with three studies identifying a positive relationship be-

tween the I allele and improved performance. Conversely,the AMPD1 34T allele and the ADRB2 16Arg allele were

associated with better performance in atmospheric hypoxia

in one study, while those with the lowest number of CAG

repeat units in the AR had better _VO2max gains following

hypoxic training in one study. This review identified that

the number of articles and the total number of polymor-

phisms studied within this area are few; only 11 articles

were identified and only 13 polymorphisms within ten

different genes studied. In addition, the conduct and/or

reporting of the studies was not optimal, with many failing

to select and describe the populations adequately, several

not conducting or describing their genotyping methodssufficiently and many conducted without calculating the

correct sample size required to adequately power the study.

4.2 Study Number and Design

The small number of articles identified in the current re-

view and the limited number of polymorphisms studied in

total highlights that the influence of genetic factors on

exercise performance in hypoxia has not been studied in

depth. This is in contrast to the hundreds of articles that

have studied the influence of genetics on other phenotypes

such as exercise performance at sea level [16]. The ma-jority of studies have taken the candidate gene approach

(n = 9), whilst two studied whether genetic admixture in-

fluenced exercise performance [13,14]. The limited range

of SNPs studied is likely due to a bias towards studying

genes associated with exercise and/or altitude phenotypes

in previous publications and to the reliance on the candi-

date gene approach. The absence of genome-wide asso-

ciation studies is notable, as it would allow hypothesis-free

investigation and help identify novel candidate genes for


1 3


12/17

Table6

Stu

dyqual

ityresu

lts

Stu

dyID

Control

group

HW

equi

librium

Casegroup/w

hole

group

Primer

Repro

duci

bility

ofgenotyping

Blind

ing

Power

calculat

ion

Statist

ics

Corrected

stat

istics

Independent

repl

icat

ion

Total

Casecontro

l

Montgomeryetal

.[15]

1

1

1

1

1

0

0

1

1

0

7/10

Djarovaetal

.[6]

0

1

1

1

0

0

1

1

0

0

5/10

Cohort:

discreteoutcome

Tsianosetal

.[10]

0

1

1

1

1

0

0

1

1

1

7/10

Thompsonetal

.[9]

0

1

1

1

1

1

0

1

1

1

8/10

Kalsonetal

.[8]

0

1

1

1

0

1

1

1

1

1

8/10

Cohort:cont

inuousoutcome

Hincksonetal

.[7]

0

1

0

0

0

0

0

1

0

2/9

Bighametal

.[5]

1

1

1

1

0

0

1

1

0

6/9

Tsianosetal

.[11]

1

1

0

1

0

1

1

1

1

7/9

Wangetal

.[12]

1a

1

1

1

0

0

1

1

0

6/9

Genet

icadm

ixture

Brutsaertetal

.[14]

1

0

1

0

0

1

1

0

4/8

Brutsaertetal

.[13]

1

0

1

0

1

1

1

0

5/8

HWHar

dyW

einburg

a

Polymorph

ism

locate

donthe

Xchromosomean

dthepopu

lationsweremale,sogenotype

distri

butionwou

ldnotconformw

ithHar

dy-W

einburgequi

librium

,thissect

ionwasscoredas

1

P. J. Hennis et al.

1 3


13/17

which the biological pathway is not known. This approach

is commonplace in many other fields of research and has

been used successfully to identify polymorphisms associ-

ated with _VO2max gains at sea level [17], obesity [18] and

blood pressure [19]. The candidate gene approach is jus-

tified, as it has greater power to detect difference in

genotype frequency between groups, and is suited to

studies using modest population sizes [20]. Despite most

studies using the candidate gene method, study designs

differed greatly with regards to the population, hypoxic

exposure, and the performance phenotype. The small

number of studies, in addition to the variety of study de-

signs, makes comparison between studies difficult, and

limits the ability to assess the reliability of results. The

variety of study designs, polymorphisms and outcomes

studied, and the small number of studies, precluded

meaningful formal assessment of bias, such as a funnel plot

[21]. That said, in spite of small sample sizes, seven of the

nine candidate gene studies identified an association be-

tween a polymorphism and the performance phenotype

with effect sizes considered very large for genetic asso-

ciation studies [22]. These factors may point towards some

form of bias within the literature base, although this cannot

be confirmed at this time.

4.3 Genetic Polymorphisms Associated with Exercise

Performance during Hypoxic Exposure

This review identified four polymorphisms associated with

exercise performance in a hypoxic environment, and found

nine polymorphisms to have no relationship (Tables3, 4,

5). Furthermore, in a study of Peruvian natives, a genetic

profile with a greater Quechua (as opposed to Spanish)

origin was associated with greater preservation of _VO2max

at altitude [14].

4.4 ACE I/D Polymorphism (rs4646994)

The ACE I/D polymorphism had the most supporting lit-

erature, with three studies observing an association of the I

allele (lower ACE activity) with mountaineering perfor-

mance [9,10,15]. ACE is an important component of the

endocrine renin-angiotensin system (RAS), converting an-

giotensin I to angiotensin II, which exerts pressor effects

through arteriolar vasoconstriction and aldosterone-medi-

ated salt and water retention [23]. However, cellular and

tissue RAS are now known to exist in multiple tissues [23],

where they have diverse roles relating to the regulation of

cell growth and survival, metabolism and inflammation

[24]. The presence (insertion, I-allele) rather than absence

(deletion, D-allele) of a 287 bp fragment in the ACE gene

(rs4646994) is associated with lower circulating [25] and

tissue [26, 27] ACE activity. Whilst the mechanism un-

derpinning the association between the ACE I/D poly-

morphism and mountaineering performance has yet to be

fully established, the I allele has been associated with en-

hanced endurance performance at sea level [28], greater

hypoxic ventilatory drive [29], enhanced arterial oxy-

genation at altitude [30], and enhanced training-related

gains in metabolic efficiency [31].It should be noted that three studies found no association

between the ACE ID polymorphism and their hypoxic

exercise performance phenotype of interest [5, 7, 8] and

one presented incorrect results [6]. The others found no

ACE genotype association with _VO2max at 4,338 m in

Peruvian natives [5] or change in endurance running per-

formance following hypoxic training [7]. However, this

later study had limited power to detect any difference in

genotype distribution due to a very small sample size.

These results are important in their own right, but it is

entirely plausible that an association exists between the

ACE polymorphism and mountaineering performance that

is mediated by a mechanism unrelated to _VO2max. The

lack of association between the polymorphism and moun-

taineering performance in the study by Kalson et al. [8] is

more difficult to explain. However, despite the altitude

exposure and performance phenotype being similar in all

four studies, there are still subtle differences in ascent rate,

maximum altitude of exposure and total time at high alti-

tude, which may explain the discrepancy. Furthermore,

Kalson et al. [8] acknowledged that they had insufficient

sample size to detect a difference between successful and

unsuccessful climbers and were therefore at risk of a type 2

error. The lack of power due to small sample size is a

limitation of many of the reviewed studies and will be

discussed further in the Study Quality section.

4.5 ACTN3 R577X (rs1815739)

The ACTN3 gene encodes the protein alpha actinin-3, an

actin-binding protein expressed predominantly in type II

(fast) muscle fibres, which has structural, cell signalling

and metabolic roles [32,33]. The ACTN3 gene contains a

nonsense polymorphism that results in the substitution of

an arginine (R) with a stop codon (X) at amino acid 577 on

the alpha actinin-3 protein (rs1815739) [34]. The prema-

ture termination of the alpha actinin-3 protein associated

with homozygosity for the R577X null allele results in

complete alpha actinin-3 deficiency, and occurs in ap-

proximately 16 % of the world population [34]. Whilst this

deficiency does not cause muscle disease, possibly due to

compensation by the closely related isoform alpha actinin-

2, it does appear to influence sporting performance [32].

The frequency of the ACTN3 alleles was reported to differ


1 3


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between a normal population, an elite sprint and power

athletic population, and an elite endurance athletic

population (p\ 0.001), with elite power athletes having a

lower frequency of the XX genotype (normal 18 %; elite

power 6 %) and elite endurance athletes having a higher

frequency of XX genotype (normal 0.16; elite endurance

0.24) [35]. The two studies identified in our review do not

support an association between the ACTN3 R577X poly-morphism and exercise performance at high altitude [6,

11]. However, given the limited performance phenotypes

investigated and the flawed statistical analysis in the study

by Djarova et al. [6], an association cannot be excluded.

4.6 AMPD1 C34T (rs17602729)

AMPD catalyses the conversion of adenosine monophos-

phate (AMP) to inosine monophosphate (IMP) and am-

monia which, in turn, moves the myokinase equilibrium

reaction (2 ADP ATP ? AMP) towards ATP re-syn-

thesis [36]. The AMPD1 gene contains a genetic variation(C34T) that results in the replacement of a glutamine

amino acid with a stop codon and has an allele frequency of

13.7 % in a healthy Caucasian population [37]. Norman

et al. [37] also showed that AMPD activity in skeletal

muscle of AMPD1 TT homozygotes was\1 % that of CC

homozygotes, and that heterozygotes had intermediate ac-

tivity levels. At sea level, the AMPD1 TT homozygotes

have been shown to have*14 % lower peak power output

than their CT/CC counterparts (p\ 0.05) [38], and the

frequency of the T allele is lower in elite endurance ath-

letes than in controls (0.043 vs. 0.085; p\ 0.05) [39].

Tsianos et al. [11] also found an association between the

AMPD T allele and Mt Olympus Marathon performance in

athletes whose preferred endurance sport was running. It is

thus unclear whether AMPD1 genotype is associated with

endurance performance per se or with hypoxic endurance

performance. No data are available to answer this question.

The results must be viewed with some caution, as they

were obtained in a sub-population of the whole group

studied and are yet to be verified in other high-altitude

exercise phenotypes (we do not consider the results of

Djarova et al. [6] valid).

4.7 Other Polymorphisms

The adrenergic receptors are G protein-coupled receptors

that are expressed in a wide range of tissues and cause a

variety of physiological responses in response to binding

with circulating catecholamines [40]. The ADRB2 sub-

group has a primary function to facilitate a sympathetic

nervous system response, altering respiratory, cardiac and

vascular function [41]. In a meta-analysis, the pooled fre-

quency of the ADRB2 minor (16Arg) allele was 0.420 in

Caucasians, 0.492 among populations with an African

heritage (African-American, African and Afro-Caribbean

populations) and 0.562 among Oriental populations [42].

The ADRB2 polymorphism alters physiological function;

compared with their ADRB2 Gly/Gly counterparts, 16Arg

homozygotes have lower receptor density on isolated

lymphocytes [43], lower resting stroke volume and cardiac

output [43, 44], and lower sub-maximal exercising strokevolume and mean arterial pressure [44]. Furthermore, the

Arg/Arg genotype was more prevalent in an elite male

endurance athlete population than in a control population

consisting of geographically matched males (0.17 vs. 0.09,

p = 0.03).

This review identified one study in which the ADRB2

Arg allele was associated with better endurance exercise

performance at altitude. Tsianos et al. [11] found the Arg

allele was associated with better performance in the Mt

Olympus Marathon amongst people whose preferred mode

of exercise was running, and with best ever Mt Olympus

Marathon time amongst those who had completed the eventseveral times. As described in the previous section re-

garding similar results for the AMPD1 C34T SNP, these

results should be viewed with caution given that these as-

sociations were established for particular subgroups and

were only sought and identified in one study. However (and

again as described above), the results are aligned with

those obtained at sea level, which increases the confidence

in the validity of such findings.

Androgens, such as testosterone, act by binding to the

AR, and their effects on anthropometric variables and

physical performance (including increasing fat free mass,

muscle strength and size) are well described [45]. In

skeletal muscle cells, the AR CAG repeat polymorphism

alters the receptors transcriptional activity, with higher

repeat lengths causing greater activity, as well as altering

markers of cell growth and development [46]. Wang et al.

[12] identified an association between those with a lower

number of CAG repeat units (B21) and a greater increase

in _VO2max following hypoxic training. However, a nor-

moxic control population was not employed and therefore

the increases in _VO2max, and the association with the AR

polymorphism, may simply reflect a training effect. Further

studies designed to distinguish the effect of training from

that of hypoxia are required.

4.8 High-Altitude Population Ancestry

The two related studies by Brutsaert and colleagues [13,

14] found no association between the degree of Quechua,

Spanish and African ancestry in a population of Peruvian

males, as indicated by 22 genetic markers and _VO2max

values at an altitude of 4,388 m. However, they did find

P. J. Hennis et al.

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that the magnitude of the decrease in _VO2max from sea

level to high altitude was lower in those with a greater

degree of Quechua ancestry. The specific mechanisms to

explain this exercise performance phenotype are not read-

ily forthcoming as the data in this field are sparse; however,

a genetically mediated reduction in arterial oxygen

saturation (SaO2) during maximal exercise appears to have

a role [14]. This finding adds to a growing body of lit-

erature suggesting that the Quechua have altered anthro-

pological and physiological responses to hypoxia as a

result of genetic selection [47].

4.9 Study Quality

Analysis of study quality highlights a need for better

conduct and/or reporting of genetic association studies in

this field. The selection of populations, genotyping prac-

tices, and statistical procedures were inadequate in many of

the studies (Table6), and only three studies attempted toreplicate prior findings using similar phenotypes [810].

Some studies did not ensure that ethnicity of populations

was accounted for in the analysis process, country of origin

often being an inadequate descriptor. Selecting unrelated

populations of the same ethnicity is critical to avoid

population stratification, which can confound results and

lead to spurious association being identified [48]. However,

such problems are hard to overcome in field research.

Poor genotyping practices may also lead to erroneous re-

sults and can be caused by many factors including col-

lecting low quality and/or quantity of DNA, machine error

or failure, human error, poor storage conditions, and issueswith the array [49]. Despite this, only eight studies stated

that they had checked the validity of their genotyping

procedure or cited a reference for a validation study [5,9

12, 14, 15], and only two blinded genotyping staff from

phenotypic data [8,9]. Meanwhile, seven studies provided

sufficient primer information to allow replication. Seven of

the 11 studies failed to conduct sample size calculations,

and only one study performed the calculation prior to

conducting the study. The majority of studies included in

the review seem underpowered to test their hypothesis ef-

fectively as the associations between SNPs and complex

human phenotypes, such as exercise performance, aregenerally small [22]. The studies in this review typically

had a sample size of under 100, whereas it has been sug-

gested that to adequately power genetic association studies

investigating complex multifactorial traits, studies should

employ sample sizes well in excess of this number [22].

Finally, replication of initial findings is of paramount im-

portance because a first study often overestimates the size

of the effect when compared with subsequent studies, and

in many cases the subsequent studies fail to support the

initial findings [50]. The studies we reviewed failed to

validate initial findings by testing the same genetic poly-

morphism on a similar population and phenotype. How-

ever, the association between the ACE polymorphism and

mountaineering performance identified by Montgomery

et al. [15] was evaluated in similar populations in three

subsequent studies, albeit using different study designs [8

10].

4.10 Strengths and Limitations of the Review

The strengths of this review are that it used systematic

search and analysis methodologies to identify studies that

have investigated the association between genetic factors

and exercise performance in atmospheric hypoxia. This

allowed us to identify all studies in the area and avoided

selection bias. Identifying these polymorphisms will guide

future research, identifying targets for further study and

helping to avoid the repetition of work. The review also

analysed the conduct and/or reporting of the studies, givingan indication of study quality and highlighting where im-

provements in study design can be made.

Limitations of this study include the method of study

selection. It may have been helpful to exclude studies

based on study design and/or quality to provide a clearer

picture of which polymorphisms and molecular systems are

important in hypoxic exercise performance. However, the

lack of a robust validated method of analysing study design

and quality in genetic association studies made this diffi-

cult. Consequently, we adapted Clark and Baudouins [4]

method of analysing study quality. This method has not

been validated and thus it is unclear whether it indicates

greater study validity, or the likelihood of replication.

However, it does follow previous studies and articles that

give recommendations for how genetic association studies

should be conducted and presented [51,52].

5 Conclusions

This review shows that the association between genetic

polymorphisms and high-altitude exercise performance has

not been studied in depth. We identified only 11 articles

that studied a total of just 13 genetic polymorphisms. Fourpolymorphisms were associated with hypoxic exercise

performance: AMPD1 C34T, ADRB2 GA, AR CAG re-

peat, and the ACE I/D polymorphisms. The strongest as-

sociation appears to be between the ACE I allele and

enhanced high-altitude mountaineering performance, with

three studies identifying a relationship. The study quality

analysis highlighted deficiencies in the conduct and re-

porting of the genetic association studies. The selection and

description of populations, genotyping methods, and


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statistical analysis procedures were inadequate in many of

the studies. In particular, many studies failed to calculate

the correct sample size required to adequately power their

study and subsequently most appear underpowered to test

their hypothesis effectively. To attain greater understand-

ing of the role that genetic factors have on high-altitude

exercise performance, further studies need to be performed.

Future studies should endeavour to use more robust studydesign (e.g. appropriate sample size, replicable phenotypes,

robust methods of selecting study and control populations)

and ensure they provide adequate information to permit

subsequent replication or meta-analysis. Furthermore,

genome-wide studies should be considered in future in

order to help identify novel biological pathways of hypoxic

adaptation.

Acknowledgments The authors would like to thank Caroline De

Brun for her invaluable help during the systematic review process.

MPWG is director of the Xtreme Everest Oxygen Research Con-

sortium and has received unrestricted grant funding from a variety of

commercial and charitable sources to support this programme of re-search. MPWG has no conflicts of interest with respect to this

manuscript and has received no financial support for the conduct of

this study or preparation of this manuscript. The other authors also

received no funding to assist in the preparation of this review and had

no potential conflicts of interest that were directly relevant to the

content of this review.

Open Access This article is distributed under the terms of the

Creative Commons Attribution License which permits any use, dis-

tribution, and reproduction in any medium, provided the original

author(s) and the source are credited.

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